The present invention relates to an electromagnetic hammer having a moving ferromagnetic mass.
Such hammers are used, for example, on building sites for driving piles in the form of stakes or sheets by percussion, and for doing so in a wide variety of ground types.
A known electromagnetic hammer comprising a tube carrying a coil and having both a moving ferromagnetic mass and an anvil in the vicinity of one of its ends is described in document JP-A-56 153 018, for example. That type of hammer presents numerous drawbacks, and the main drawback is the lack of any rigid support for the coil, such that while the mass is being raised, said coil is subjected to a considerable reaction force causing it to become compacted. In use, these successive deformations of the coil cause the performance of the electromagnetic hammer to diminish and can lead to the coil being damaged.
Document U.S. Pat. No. 5,168,939 discloses a device for drilling an oil well with an electromagnetically accelerated impactor, the device comprising a plurality of coil modules separated from one another merely by spacers and stacked one on another in a carrier structure Building up the coil as a plurality of independent modules makes it possible to control the electromagnetic force generated by each module. The stack of coil modules is prestressed so as to prevent the modules separating from one another in use, particularly under the effect of the electromagnetic reaction as the impactor goes past. The impactor is inserted manually into the top of the device so that no provision is made for it to be raised by means of an electromagnetic force generated by the coil modules. The problem of the modules withstanding the compression induced by the electromagnetic reaction while the impactor is being raised is not addressed in that document even though that problem constitutes a major weakness for such a device whose modules can deteriorate rapidly. That document does not address questions of sealing, either.
For technological background, reference can also be made to the following documents: U.S. Pat. No. 4,799,557, U.S. Pat. No. 4,468,594, and U.S. Pat. No. 4,215,297.
More recently, proposals have been made for a higher-performance electromagnetic hammer in which the coil is made by being wound around the hammer tube, said tube being made of a non-magnetic material and having means for taking up axial forces and for transmitting said forces to the anvil while the mass is being raised.
One such electromagnetic hammer is described in document FR-A-2 765 904 assigned to the Applicant. In a particular embodiment, provision is made for an additional coil made by winding around the same central tube at an axial position situated between the coil and the anvil, said additional coil being connected to the main coil so as to be powered by the current induced therein as the mass travels downwards.
Nevertheless, winding the coil directly on the tube for the electromagnetic hammer as described above presents certain drawbacks that are explained below.
Using a one-piece internal tube whose length is about 4 meters (m) to 5 m means that it cannot be impregnated with an electrical varnish since the length of such a tube greatly exceeds the capacity of the impregnation baths that are conventionally used. Consequently, the internal tube of the electromagnetic hammer is relatively vulnerable to moisture, and to mechanical jamming due to the tube swelling. Furthermore, it has been found that the bottom portion of the coil wound directly on the tube is very highly stressed in use. As an indication, the compression force on the coil while the mass is being raised corresponds to a force of about 20 (metric) tonnes. As a general rule, the coils used are made by winding a conductor whose section is in the form of a rectangular flat extending in the height direction. Consequently, very high pressure exerted vertical on the windings of the coil run the risk of giving rise to plastic deformation of the coil material (generally copper). The effect of this deformation is to crush the insulation concerned, which leads progressively to turns becoming short-circuited one to another. The phenomenon amplifies quickly since the reduction in electrical resistance gives rise to an increase in temperature rise and consequently to the insulation being destroyed by short-circuiting or by overheating. Finally, it has been found that the above-described electromagnetic hammer structure is relatively vulnerable to moisture due to it being very difficult to make the coil waterproof. Under such circumstances, if the coil becomes damaged, it is necessary to stop using the electromagnetic hammer and then to remove the coil from the central tube, and that can only be done with equipment that is heavy and bulky, giving rise to the drawback of a prolonged interruption in work.
The present invention seeks to resolve the above-mentioned problem by designing an electromagnetic hammer that does not suffer from the above drawbacks or limitations, while nevertheless conserving the advantages of the structure described in above-mentioned document FR-A-2 765 904.
This problem is solved by the invention by means of an electromagnetic hammer having a moving ferromagnetic mass, the hammer being of the type comprising a tube of non-magnetic material for standing on an element that is to be driven into the ground, said tube being surrounded by a peripheral coil connected to electrical power supply means and slidably receiving the moving mass, the hammer being remarkable in that the peripheral coil is subdivided into a plurality of independent coils, each independent coil being received in an associated casing and being wound around a cylindrical inner wall of said casing, the cylindrical inner walls of the casings being superposed to make up the tube in which the moving mass slides, each casing also having means for taking up axial forces, and a junction box enabling the corresponding coil to be connected to associated electrical power supply cables.
Making the coil as a plurality of independent coils received in associated casings makes it possible to distribute the forces exerted on the bottom of each coil, so that the total force to be withstood is divided by the number of independent coils used. This ensures that the stress applied to the windings of each coil is limited to a considerable extent while avoiding the risk of the insulation being crushed and the coil being short-circuited.
Preferably, each coil is received in watertight manner in its associated casing, the reception housing being defined by end rings and by a cylindrical outer wall. In particular, each coil is held inside its reception housing by a filler resin. Thus, each winding is well protected against external attack, and the electromagnetic hammer can be operated in surroundings that are very wet.
It is then advantageous for each casing to be defined by a bottom ring and a top ring, one of which is an end ring defining the reception housing for the coil, and the other of which is disposed at a distance from the other end ring, with reinforcing spacers being interposed, said rings and spacers constituting said means for taking up axial forces. Securing the coil inside the associated casing serves both to take up and to limit the axial compression forces on the windings concerned.
Also advantageously, the junction box of each casing is external and waterproof.
It is then preferable for each junction box to be disposed between two rings of the casing, and to comprise a waterproof housing associated with the inlet and outlet connections of the coil and from which there projects a terminal box receiving elements for connection to the corresponding cables. In particular, the waterproof housing associated with the junction box is filled with a coating material, in particular liquid silicone. The use of such external boxes enables each coil to be tested separately and enables any faults to be identified.
Also preferably, the superposed casings are interconnected by releasable connections, in particular by bolt fastenings, so that each casing is individually interchangeable. In particular, the coils are arranged to enable said hammer to operate in an impaired mode in the event of one of the coils being damaged. Thus, when a fault is detected, the defective coil can be taken electrically out of service by an external modification to the cabling, without that interrupting operation of the hammer which then continues to operate with impaired performance, i.e. with voltages and hammering rates that are reduced. In addition, in the event of a fault in a casing, the external mechanical and electrical accessibility makes it possible for site personnel to swap casings quickly, thereby avoiding the need to take the production tool out of operation for too long.
Because of its modular structure, it is then possible to repair the damaged casing on its own, thereby reducing the cost and the time required for reconditioning.
In a particular embodiment, the n coils are electrically connected in series or in parallel, with the 2n corresponding cables being connected to a connection bar junction box. These connection systems make it possible to bypass a damaged coil very quickly.
In a variant, provision can be made for the n coils to be electrically connected in series, with the two corresponding cables being connected to an external junction box.